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Charging lithium-ion
batteries

There
is only one way to charge lithium-based batteries. The so-called
'miracle chargers', which claim to restore and prolong batteries, do not
exist for lithium chemistries. Neither does super-fast charging apply.
Manufacturers of lithium-ion cells have very strict guidelines in charge
procedures. Most cells are charged to 4.20 volts with a tolerance of
?0.05V/cell. Charging only to 4.10V reduced the capacity by 10% but
provides a longer service life. Newer cell are capable of delivering a
good cycle count with a charge to 4.20 volts per cell. Figure 1 shows
the voltage and current signature as the lithium-ion cell passes through
the charge stages.

Figure 1: Charge stages of
a lithium-ion battery. Increasing the charge current on a lithium? ion
charger does not shorten the charge time by much. Although the voltage
peak is reached quicker with higher current, the topping charge will
take longer.

The charge time of most
chargers is about 3 hours. Smaller batteries used for cell phones can be
charged at 1C; the larger 18650 cell used for laptops should be charged
at 0.8C or less. The charge efficiency is 99.9% and the battery remains
cool during charge. Full charge is attained after the voltage threshold
has been reached and the current has dropped to 3% of the rated current
or has leveled off.

Increasing the charge
current does not shorten the charge time by much. Although the voltage
peak is reached quicker with higher charge current, the topping charge
will take longer.

Some chargers claim to
fast-charge a lithium-ion battery in one hour or less. Such a charger
eliminates stage 2 and goes directly to 'ready' once the voltage
threshold is reached at the end of stage 1. The charge level at this
point is about 70%. The topping charge typically takes twice as long as
the initial charge.

No trickle charge is
applied because lithium-ion is unable to absorb overcharge. A continuous
trickle charge above 4.05V/cell would causes plating of metallic lithium
that could lead to instabilities and compromise safety. Instead, a brief
topping charge is provided to compensate for the small self-discharge
the battery and its protective circuit consume. Depending on the
battery, a topping charge may be repeated once every 20 days. Typically,
the charge kicks in when the open terminal voltage drops to 4.05V/cell
and turns off at a high 4.20V/cell.

What happens if a battery
is inadvertently overcharged? lithium-ion is designed to operate safely
within their normal operating voltage but become unstable if charged to
higher voltages. When charging above 4.30V, the cell causes plating of
metallic lithium on the anode; the cathode material becomes an oxidizing
agent, loses stability and releases oxygen. Overcharging causes the cell
to heat up. If left unattended, the cell could vent with flame.

Much attention is focused
to avoid over-charging and over-discharging. Commercial lithium ion
packs contain a protection circuits that limit the charge voltage to
4.30V/cell, 0.10 volts higher than the voltage threshold of the charger.
Temperature sensing disconnects the charge if the cell temperature
approaches 90ºC, and a mechanical pressure switch on many cells
permanently interrupt the current path if a safe pressure threshold is
exceeded. Exceptions are made on some spinel (manganese) packs
containing one or two small cells.

Extreme low voltage must
also be prevented. The safety circuit is designed to cut off the current
path if the battery is inadvertently discharged below 2.50V/cell. At
this voltage, most circuits render the battery unserviceable and a
recharge on a regular charger is not possible. There are several
safeguards to prevent excessive discharge. The equipment protects the
battery by cutting off when the cell reaches 2.7 to 3.0V/cell. Battery
manufacturers ship the batteries with a 40% charge to allow some
self-discharge during storage. Advanced batteries contain a wake-up
feature in which the protection circuit only starts to draw current
after the battery has been activated with a brief charge. This allows
prolonged storage.

In spite of these
preventive measures, over-discharge does occur. The Cadex C7200 and
C7400 battery analyzers feature a 'boost' function that provides a
gentle charge current to activate the safety circuit and re-energize the
cells if discharged too deeply. A full charge and analysis follows.

If the cells have dwelled
at 1.5V/cell and lower for a few days, however, a recharge should be
avoided. Copper shunts may have formed inside the cells, leading a
partial or total electrical short. The cell becomes unstable. Charging
such a battery would cause excessive heat and safety could not be
assured.

Battery experts agree that
charging lithium-ion batteries is simpler and more straightforward than
the nickel-based cousins. Besides meeting the tight voltage tolerances,
the charge circuit can be designed with fewer variables to consider.
Full-charge detection by applying voltage limits and observing the
current saturations on full charge is simpler than analyzing many
complex signatures, which nickel-metal-hydride produces. Charge currents
are less critical and can vary. A low current still permits proper full
charge detection. The battery simply takes longer to charge. The absence
of topping and trickle charge also help in simplifying the charger. Best
of all, there is no memory but aging issues are the drawback.

The charge process of a
lithium-ion-polymer is similar to lithium-ion. These batteries use a
gelled electrolyte to improve conductivity. In most cases, lithium-ion
and lithium-ion-polymer share the same charger.